1. Field of the Invention
The present invention relates generally to a switching signal modulating circuit for use in switching control of a switching regulator, etc., and more particularly, to a switching signal modulating circuit that varies randomly the frequency and the duty of a switching signal using a spread signal.
2. Description of the Related Art
Conventionally, a switching regulator is adapted to produce a control signal for frequency modulation of a triangular wave signal generating circuit by detecting the maximal value of a triangular wave signal and sampling a random signal at the timing of the detected maximal value in order to reduce the peak levels of the conduction noise and the radiation noise accompanied by switching control, is adapted to obtain a PMW signal by varying randomly the triangular signal by a spread width of 0.8 to 1.2 times as wide as that of the central frequency and by comparing this triangular signal with a reference voltage using a comparator and is adapted to control the turning on and off of a switching device (see, e.g., Japanese Patent Application Laid-Open Publication Nos. 1995-264849 and 2003-274656).
However, such a conventional switching signal modulating circuit as above needs a peak detecting circuit and a sample holding circuit and has a problem that a transformer is saturated because the width of an ON time period is increased when the switching frequency is lowered due to the frequency spread. Furthermore, as to the above switching signal modulating circuit, the circuitry is complicated, designing of the logic and adjustment take time and the control circuit becomes bulky. To use a dedicated IC can be considered in order to solve the problems. However, this needs much time to deliver and a higher cost. In addition, other problems also arise that the scope of the designing to reduce the spread frequency is limited by the restriction on the size of the transformer, etc.